if you merge the fields there should be no loss of quality beyond the smear, blur or ghosting that results from the movement between the fields. if you look at an interlaced frame on an interlace capable monitor you will see it "jitter" between the 2 fields, you cannot see that on a progressive monitor. some computer video cards now hide the flicker - BlackMagic give an option for only showing one field when stopped as the jitter annoys some people. when you field merge the jitter stops as the 2 fields are now the same. field merging is not setting dominance correctly it is summing the 2 fields. it was fashionable for a while in the late eighties/early nineties to field "strip" to get a "film" look as it reduced the 50hz movement resolution to the 25 of film (in PAL countries if people were shooting film for TV they shot 25fps because it was such an easy fit for PAL - no pulldown!) but field stripping resulted in a loss of resolution as it was only field 1 or field 2 that was copied to both fields. field dominance isnt such an issue as it was in the DV days as it had lower/even dominance as a result of the type of compression usedin that format. when field dominance is wrong it reverses the motion for that frame - sort of like 2 steps forward, one step back.
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